Various systems have been proposed for mooring vessels such as tankers at offshore locations and transferring crude oil or other fluids between a submarine pipeline and the manifold on the deck of the vessel. Some are relatively simple but are not capable of unlimited weathervaning. Others have unlimited weathervaning capability but involve heavy and sophisticated structures and have a relatively high capital cost. Some are difficult for the vessel to pick up and disconnect. Some involve extensive traumatic invasion of the vessel hull such that the vessel cannot readily thereafter be reassigned to ordinary ocean transport duty. They also involve long and expensive drydock time.
U.S. Pat. No. 5,944,448 and GB 2,296,904 describe mooring and flowline systems which comprise a three-leg mooring and flexible riser. The flexible riser is without rotational couplings, and has a part of its length secured to a mooring pendant. There is therefore a restriction on the number of turns the vessel can make, since turning full circle will effectively twist the fluid riser in the chafe chains around one another. The systems of U.S. Pat. No. 5,944,448 and GB 2,296,904 therefore have limited weathervaning capabilities and are prone to fatigue and wear problems.
GB 2,359,054 describes a similar system in which a riser is secured to a non-swivelling node 18 and a mooring pendant. The riser comprises a single rotational coupling. As with the systems of U.S. Pat. No. 5,944,448 and GB 2,296,904, the arrangement of GB 2,359,054 is designed to cause the riser pipe to helix around the pendant chain, restricting weathervaning capabilities and inducing fatigue and wear.
Internal turret mooring systems consist of a turret and a turret casing integrated into the hull of a vessel. The two parts are connected via a bearing system which allows the turret casing to rotate around the turret. A typical design of an internal turret mooring system enables the connection of risers and associated umbilicals via a swivel stack. GB 2285028 is an example of a disconnectable turret mooring system integrated into the bow of a vessel, and WO 03/039946 is an example of a turret for the connection of a buoy to a vessel.
Internal turret systems such as those described above are in common use and are an effective means for enabling mooring and fluid transfer with full weathervaning. However, internal turret systems are expensive to implement with capital expenditure often in excess of $30m for turret fabrication and integration into the vessel.
EP 0656293 describes an alternative internal turret vessel mooring system. The document also describes a configuration in which the turret casing is mounted on a structure which extends beyond the bow of the vessel such that the turret is external to the hull (see FIG. 1 of EP 0656293). EP 1796958 is another example of an offshore vessel mooring and riser inboarding system which offers similar functionality to an internal turret but via an external assembly. In this case, the system comprises a cantilever support mounted on a bow of the vessel. This system provides a gimbal arrangement which enables movement of a turret about three mutually perpendicular axes.
External turret arrangements of a type described in EP 1796958 and EP 0656293 have the advantage that their implementation is less invasive but they still have high capital expenditure which renders them unsuitable for some installations (including short- and medium-term installations).
WO 96/11134 describes a Submerged Catenary Anchor Leg Mooring (CALM) buoy system. The CALM buoy is arranged to float below sea-level, and is anchored to the seabed by catenary anchor lines. The buoy comprises an upper and lower part, and a turntable to allow the mooring pendants to pivot with respect to the anchors. An upper hose is connected to the upper part of the buoy, and lower hoses are connected to the lower part of the buoy.
WO 2011/042535 describes another CALM mooring buoy system including a swivel. Anchor lines are connected to a lower part of the buoy beneath the swivel, and mooring lines are attached to an upper part of the buoy above the swivel. Riser terminations are provided on upper and lower parts of the buoy, with a fluid swivel arranged between the respective upper and lower risers.
The systems of WO 96/11134 and WO 2011/042535 require structural buoyancy, which is submerged in the case of WO 96/11134; the CALM buoys comprise several mechanical parts, which increases complexity and has significant implications for fabrication and installation costs.
U.S. Pat. No. 3,979,785 describes a single point mooring system comprising a mooring buoy and an anchor hub. The anchor hub is moored by catenary anchor legs, and the anchor hub is connected to the mooring buoy via a chain and swivel which allows the mooring buoy to rotate relative to the anchor hub. A cargo transfer swivel connects an underwater cargo hose to a bifurcated hose arm leading to a vessel manifold.
The system of U.S. Pat. No. 3,979,785 requires dedicated equipment and specialised assembly. It is not possible to use the system of U.S. Pat. No. 3,979,785 with a continuous riser; a fluid path swivel is necessary to provide fluid connections at the node.
WO 00/51881 discloses a single point mooring system in which a mooring line and a loading hose swivel about an anchor point on the seabed. A lump weight fastens the mooring line to the loading hose to form a point of division between a lower part and an upper part of the hose. The systems of U.S. Pat. No. 3,979,785 and WO 00/51881 are relatively complex to install. In addition, the designs limit the number of chains that can be brought directly to the vessel bow, which may compromise the robustness of the moorings.
There is a need in the market for a simple, robust and economical system which lends itself to use in the context of short term FSO installations such as those serving Extended Well Test export systems (EWTs) as well as in the context of medium term installations supporting Early Production Systems (EPSs) and in the context of longer term installations supporting Floating Production Storage and Offtake systems (FPSOs) and Floating Storage and Offtake systems (FSOs) and Shuttle Tanker Loading Systems.
It is amongst the objects of the invention to provide a mooring system and/or a connector assembly which obviates at least mitigates one or more deficiencies of previously proposed mooring systems, and in which mariners can have confidence. One aim of the invention is provide a mooring system and/or a connector assembly that has a good weathervaning capability. Another aim of the invention is to provide a mooring system and/or a connector assembly which facilitates quick and efficient disconnect and reconnect operations; is easy to install and recover; is easy and efficient to use; and/or has relatively low capital and operating costs.
Further aims and objects of the invention will become apparent from reading the following description.